Engines, including diesel engines, gasoline engines, natural gas engines, and other engines known in the art, may exhaust a complex mixture of air pollutants. The air pollutants may include both gaseous and solid material, such as, for example, particulate matter. Particulate matter may include ash and unburned carbon particles and may sometimes be referred to as soot.
Due to increased environmental concerns, among other things, exhaust emission standards may have become more stringent. The amount of particulate matter and gaseous pollutants emitted from an engine may be regulated depending on the type, size, and/or class of engine. In order to meet these emissions standards, engine manufacturers have pursued improvements in several different engine technologies, such as fuel injection, engine management, and air induction, to name a few. In addition, engine manufacturers have developed devices for treatment of engine exhaust after it leaves the engine.
Engine manufacturers have employed exhaust treatment devices called particulate traps to remove the particulate matter from the exhaust flow of an engine. A particulate trap may include a filter designed to trap particulate matter. The use of the particulate trap for extended periods of time, however, may enable particulate matter to accumulate on the filter, thereby causing damage to the filter and/or a decline in engine performance. Systems have been developed to measure, or estimate, an amount of particulate matter that has accumulated on the filter.
For example, one such system for soot detection is described in U.S. Patent Publication No. US2001/0051108 (the publication) by Schonauer. The publication describes an apparatus for detecting soot. The apparatus includes a porous ceramic element, a heater, a first temperature probe, and a second temperature probe. The heater applies a constant level of heat to the porous ceramic element that is equivalent to the minimum ignition temperature of soot. As exhaust gas containing soot passes through the pores of the ceramic element, the exhaust gas is heated to the soot ignition temperature combusting the soot and further increasing the temperature of the exhaust gas. Such a temperature increase is measured by the first temperature probe. At the same time, a portion of the exhaust gas is directed to a chamber through a filter, which filters most of the soot contained within the exhaust gas. Because the exhaust gas in the chamber contains such a small amount of soot, the temperature of exhaust gas in the chamber provides a baseline temperature for comparison with the temperature increase measured by the first temperature probe. Such a baseline temperature is measured by the second temperature probe. The difference between the two temperatures is directly related to the amount of soot contained within the exhaust gas.
Although the apparatus described in the publication may detect the amount of soot contained within the exhaust gas, the design may be prone to inaccuracies. In particular, because the exhaust gas is heated to the ignition temperature of soot before reaching both temperature probes, any soot that is allowed to reach the second temperature probe may compromise soot level measurements. To isolate the second temperature probe from soot contained within the exhaust gas, a filtering element and an isolation chamber must be built within the sensor housing. Inherent manufacturing inconsistencies may limit the filtering capability of any filtering element, and unknown levels of soot may infiltrate the isolation chamber. These manufacturing inconsistencies may limit the accuracy of any soot level measurement by distorting the baseline temperature due to undesired soot combustion.
In addition, the apparatus described in the publication may be limited to only the detection of soot because it relies on heating the exhaust gas to the ignition temperature of soot. However, it may be desired to detect the level of other types of pollutants contained within the exhaust gas. Preheating the exhaust gas to determine the level of such pollutants may not be preferred because the pollutants may have ignition temperatures that may be difficult to attain or may damage other equipment in the exhaust system.
The disclosed apparatus is directed to overcoming one or more of the problems set forth above.